Fuel cells, oxygen sensors, oxygenation membrane devices, and the like have an electrolyte-electrode joined assembly produced by forming an anode and a cathode on surfaces of an oxide ion-conductive electrolyte. In such an assembly, oxygen is ionized on the cathode, and generated oxide ions are transferred through the electrolyte to the anode.
The transfer of oxide ions is activated at a relatively high temperature. The electrolyte-electrode joined assembly must be heated to such a high temperature in order to drive the fuel cell, etc. Thus, the fuel cell cannot produce electric power immediately as needed.
Accordingly, electrolytes composed of substances excellent in oxide ion conductivity, even at relatively low temperatures, have been proposed in recent years. Specifically, such substances include fluorite-type oxides such as scandium-stabilized zirconias (ScSZ) and samarium-doped cerias (SDC), perovskite-type oxides such as lanthanum gallate (LaSrGaMgO), etc. In addition, the applicant has proposed an electrolyte-electrode joined assembly having an electrolyte composed of an apatite-type composite oxide (see, e.g., Patent Document 1).
In these types of electrolytes, it is expected that oxide ion conductivity and electric conductivity can be further increased by reducing the thickness thereof as small as possible. In this case, the oxide ion transfer distance is shortened, and ohmic loss is reduced.
As described in Patent Document 2, in a so-called self-supporting electrolyte-electrode joined assembly, which is produced by forming an electrolyte first and then forming an anode and a cathode on surfaces of the electrolyte, the electrolyte is thicker than the anode and the cathode. When the thickness of the electrolyte is reduced, and thicknesses of the electrodes are excessively reduced, the resultant assembly is poor in strength. Thus, when the thickness of the electrolyte is reduced, the electrodes are made relatively thick so as to obtain sufficient strength.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-149795    Patent Document 2: Japanese Laid-Open Patent Publication No. 09-190825